Liquid crystal display device having drive circuit

ABSTRACT

A fabricating method of an array substrate for a liquid crystal display device including forming a polycrystalline silicon film on a substrate having a display region and a peripheral region, the polycrystalline silicon film having grains of square shape, forming a first active layer in the display region and a second active layer in the peripheral region by etching the polycrystalline silicon film, forming a first gate electrode over the first active layer, a second gate electrode over the second active layer and a gate line connected to the first gate electrode, and forming first source and drain electrodes connected to the first active layer, second source and drain electrodes connected to the second active layer and data line connected to the first source electrode. Further, the second gate electrode overlaps the first active layer to form a first channel region, and the first channel region is formed inside one of the grains.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of U.S. application Ser. No. 11/984,031filed Nov. 13, 2007 now U.S. Pat. No. 7,674,664, which is a divisionalof U.S. application Ser. No. 10/745,614 filed Dec. 29, 2003 now U.S.Pat. No. 7,312,471, which claims the benefit of the Korean ApplicationNo. P2002-87302 filed on Dec. 30, 2002, the entire contents of which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a liquid crystal display device, and moreparticularly, to a liquid crystal display device that includes a drivingcircuit composed of polycrystalline silicon thin film transistors.

2. Discussion of the Related Art

Liquid crystal display (LCD) devices are developing as the nextgeneration of display devices because they are portable and consumelittle power. In general, an LCD device includes two substrates disposedsuch that respective electrodes of the two substrates face into eachother. A liquid crystal layer is interposed between the respectiveelectrodes. When a voltage is applied to the electrodes, an electricfield is generated. The electric field modulates the light transmittanceof the liquid crystal layer by reorienting the liquid crystal molecules,thereby displaying images in the LCD device.

One substrate of an LCD device includes a thin film transistor (TFT)that acts as a switching device. The TFT is often formed using amorphoussilicon as an active layer. One reason for this is that amorphoussilicon can be formed on a large, low cost substrate such as glass. LCDdevices also include drive integrated circuits (drive ICs) that controlthe TFT. Unfortunately, amorphous silicon does not form a suitableactive layer for drive ICs, which are usually CMOS (complementarymetal-oxide-semiconductor) devices that require an active layer ofsingle crystalline silicon. Because of this, drive ICs are usuallyconnected to a TFT substrate using a TAB (tape automated bonding)system. This adds significant cost to LCD devices.

Because of the limitations of amorphous silicon, LCD devices thatincorporate a polycrystalline silicon TFT, in which polycrystallinesilicon is used as an active layer, are under research and development.Polycrystalline silicon is highly preferred because it is better suitedfor use in a drive IC than amorphous silicon. Polycrystalline siliconthus has the advantage that the number of fabrication steps can bereduced because thin film transistors and drive ICs can be formed on thesame substrate, thus eliminating the need for TAB bonding. Furthermore,the field effect mobility of polycrystalline silicon is 100 to 200 timesgreater than that of amorphous silicon. Polycrystalline silicon is alsooptically and thermally stable.

Among many recent methods of forming polycrystalline silicon, a newmethod of crystallization, often referred to as sequential lateralsolidification (SLS), has become of interest. The SLS method takesadvantage of the fact that silicon grains grow laterally from the phaseboundary between liquid silicon and solid silicon. The SLS method canincrease the size of the silicon grains by controlling the energyintensity of a laser beam and the irradiation range of the laser beamused to grow the silicon grains.

FIG. 1 shows a schematic view of an apparatus for the related art SLSmethod.

FIG. 1 depicts an apparatus for the SLS method that includes a lightsource 1, an attenuator 3, a focusing lens 9, a mask 11, an imaging lens13, and a moving stage 19 on which a sample 17 having an amorphoussilicon layer 20 (of FIG. 2A) is situated. The apparatus for the SLSmethod also includes first to third reflective mirrors 5, 7, and 15 tochange the direction of the light. The first and second reflectivemirrors 5 and 7 are disposed between the attenuator 3 and the focusinglens 9, and the third reflective mirror 17 is disposed between theimaging lens 13 and the moving stage 19.

The light source 1 is preferably a XeCl (xenon-chloride) excimer laserhaving a wavelength of 308 nm. The attenuator 3 controls the energy ofthe laser beam through the system. The focusing lens 9 and the imaginglens 13 condense the laser beam, while the focusing lens 9 makes theintensity of the laser beam more uniform by equalizing focus lengths ofthe laser beam. The mask 11 forms the laser beam into a predeterminedshape.

The laser beam from the light source 1 therefore transmits through theattenuator 3 and is reflected by the first and second reflective mirrors5 and 7. The laser beam is then condensed by the focusing lens 9, shapedby the mask 11, and passes through the imaging lens 13. The laser beamis next reflected by the third reflective mirror 15 onto the sample 17.The moving stage 19 then moves the sample 17, and irradiation isrepeated.

FIGS. 2A to 20 show schematic plane views depicting a process forcrystallizing an amorphous silicon film using the related art apparatusfor the SLS.

In FIG. 2A, a first laser beam irradiation has been carried out at afirst region “A” of an amorphous silicon film 20. As discussed above,the silicon grains grow laterally from the boundary between liquid phasesilicon and solid phase silicon to result in first grains 22 growingfrom both sides of the first region “A.” Grain growth stops at a firstline “IIa” where the first grains 22 meet.

FIG. 2B shows the results of when a second laser beam irradiation iscarried out at a second region “B” of the amorphous silicon film 20. Thesecond region “B” includes part of the first region “A.” In a firstoverlapping region “AB,” where the first region “A” and the secondregion “B” overlap, the grains 22 (of FIG. 2A) act as crystallizationseeds. Grain growth stops at a second line “IIb” where second grains 23meet. The second grains 23 are larger than the first grains 22, whichwere formed after the first laser beam irradiation.

FIG. 2C shows a third laser beam irradiation accomplished at a thirdregion “C” of the amorphous silicon film 20. Third grains 24 grow fromboundaries of the third region “C”. The third region “C” includes partof the second region “B.” In the second overlapping region “BC,” wherethe second region “B” and the third “C” region overlap, the secondgrains 23 (of FIG. 2B) act as crystallization seeds. The third grains 24are therefore much larger than the second grains 23 (of FIG. 2B).

Repeated laser beam irradiation scans the whole amorphous silicon film20 to create polycrystalline silicon with large grains. Furthermore,high crystallization productivity results from the small number of timesthe same point is irradiated.

However, a polycrystalline silicon film formed by the SLS method tendsto have different-sized grains and irregular growing directions. Thus,TFTs fabricated from the polycrystalline silicon film by the SLS methodalso have properties that depend on the grain-growth direction and grainboundary.

FIG. 3 shows a schematic graph showing a property of a thin filmtransistor using a polycrystalline silicon film grown by the related SLSmethod.

In FIG. 3, the x-axis indicates the TFT gate voltage (Vg) and the y-axisindicates the TFT drain current (Id). Each line also shows a first case(solid lines) in which the directions of the channel-passed current andthe grain-growth are parallel, a second case (short dotted lines) inwhich the directions of the channel passed current and the grain-growthdirection form an angle of 45 degrees, and a third case (long dottedlines) in which the directions of the channel passed current and thegrain-growth have an angle of 90 degrees. These lines are established atdrain voltages “Vd” of 0.1 V and 10 V. FIG. 3 shows that reducing theangle between the channel direction and the grain-growth directiondecreases the number of the grain boundaries. This improves thecurrent-voltage characteristics. Therefore, if the channel direction ofa TFT runs parallel with the grain-growth direction, then the TFTproperties are enhanced. However, if the channel direction of a TFT andthe grain-growth direction are at an angle of 90 degrees, then theproperties of TFT are minimized.

FIGS. 4 and 5 show schematic plane views depicting thin film transistorsformed using a polycrystalline silicon film made by the SLS methodaccording to first and second related art embodiments, respectively.FIGS. 4 and 5 have TFTs with a grain size of the polycrystalline siliconfilm smaller than a channel area “ch” of the TFT.

The current path of the channel area “ch” and the grain-growth directionmake an angle of 90 degrees In FIG. 4, while the current path isparallel to the grain-growth direction in FIG. 5. Since the TFT of FIG.5 is less influenced by the grain boundary than that of FIG. 4, the TFTof FIG. 5 has superior characteristics to that of FIG. 4. A TFT using apolycrystalline silicon film by the SLS method accordingly has optimalcharacteristics when the current path through the channel coincides withthe grain-growth direction, and the TFT has the most inferiorcharacteristics when the current path through a channel is perpendicularto the grain-growth direction. Thus, when a polycrystalline silicon filmmade by the related art SLS method is used for a TFT, a uniform propertyof the TFT can not be obtained. Moreover, even when the best TFT isobtained, the characteristics of the TFT are not sufficient for a driveIC, but merely for a gate driver and a data driver.

SUMMARY OF THE INVENTION

Accordingly, the invention is directed to a liquid crystal displaydevice and a fabricating method thereof that substantially obviates oneor more of problems due to limitations and disadvantages of the relatedart.

An object of the invention is to provide a liquid crystal display devicethat includes a drive circuit using a polycrystalline silicon film, anda fabricating method thereof.

An object of the invention is to provide a fabricating method of aliquid crystal display device in which a thin film transistor is formedsuch that one grain includes at least one channel area.

The invention, in part, pertains to an array substrate for a liquidcrystal display device that includes a substrate having a display regionand a peripheral region; a gate line on the substrate; a data linecrossing the gate line; a switching thin film transistor connected tothe gate line and the data line; a gate driver connected to the gateline; a data driver connected to the data line; and a drive circuitconnected to the gate driver and the data driver, wherein the gate line,the data line and the switching thin film transistor are formed in thedisplay region, and the gate driver, the data driver and the drivecircuit are formed in the peripheral region. The drive circuit includesa drive thin film transistor, and each of the switching thin filmtransistor and the drive thin film transistor includes an active layer,a gate electrode, and source and drain electrodes, wherein the activelayer includes polycrystalline silicon having square shaped grains.Also, the gate electrode overlaps the active layer to form a channelregion, wherein the channel region of the drive thin film transistor isformed inside one of the grains.

The invention, in part, pertains to a fabricating method of an arraysubstrate for a liquid crystal display device that includes forming apolycrystalline silicon film on a substrate having a display region anda peripheral region, the polycrystalline silicon film having squareshaped grains; forming a first active layer in the display region and asecond active layer in the peripheral region by etching thepolycrystalline silicon film; forming first gate electrode over thefirst active layer, a second gate electrode over the second active layerand a gate line connected to the first gate electrode; and forming firstsource and drain electrodes connected to the first active layer, secondsource and drain electrodes connected to the second active layer anddata line connected to the first source electrode, wherein the secondgate electrode overlaps the first active layer to form a first channelregion, and the first channel region is formed inside one of the grains.

The invention, in part, pertains to a thin film transistor for a liquidcrystal display device that includes a substrate; an active layer on thesubstrate, the active layer includes polycrystalline silicon havingsquare shaped grains; gate electrode overlapping the active layer toform a channel region, the channel region being formed inside one of thegrains; and source and drain electrodes connected to both sides of theactive layer.

The invention, in part, pertains to a fabricating method of a thin filmtransistor for a liquid crystal display device that includes forming apolycrystalline silicon film on a substrate, the polycrystalline siliconfilm having square shaped grains; forming an active layer by etching thepolycrystalline silicon film; forming a gate electrode over the activelayer, the gate electrode overlapping the active layer to form a channelregion, the channel region being formed inside one of the grains; andforming source and drain electrodes connected to both sides of theactive layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 shows a schematic view showing an apparatus for the related artSLS method.

FIGS. 2A to 2C show schematic plane views showing a related art processof crystallizing an amorphous silicon film using the related artapparatus for the SLS method.

FIG. 3 shows a schematic graph showing a property of a thin filmtransistor using the related art polycrystalline silicon film formed bythe related art SLS method.

FIG. 4 shows a schematic plane view depicting a thin film transistorusing a polycrystalline silicon film made by the SLS method according toa first embodiment of the related art.

FIG. 5 shows a schematic plane view depicting a thin film transistorusing a polycrystalline silicon film made by the SLS method according toa second embodiment of the related art.

FIG. 6 shows a schematic configuration depicting a mask for a sequentiallateral solidification (SLS) method according to an embodiment of theinvention.

FIGS. 7A to 7D show schematic plane views depicting a process ofcrystallizing an amorphous silicon film according to an embodiment ofthe invention.

FIG. 8 shows a schematic view depicting a position of a thin filmtransistor according to a first embodiment of the invention.

FIG. 9 shows a schematic view of a position of a thin film transistoraccording to a second embodiment of the invention.

FIG. 10 shows a schematic plane view of an array substrate for a liquidcrystal display device according to an embodiment of the invention.

DETAILED DESCRIPTION

Features and advantages of the invention will be set forth in thedescription which follows and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

Reference will now be made in detail to the illustrated embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are usedthroughout the drawings to refer to the same or like parts.

FIG. 6 shows a schematic configuration of a mask for a sequentiallateral solidification (SLS) method according to an embodiment of theinvention.

In FIG. 6, the mask 100 includes first to fourth regions 110, 120, 130,and 140. The first region 110 has multiple first stripes 111 separatedby multiple first slits 112. The second region 120 has multiple secondstripes 121 separated by multiple second slits 122. The third region 130has multiple third stripes 131 separated by multiple third slits 132,with the third strips 131 being aligned with the first slits 112. Thefourth region 140 has multiple fourth stripes 141 separated by multiplefourth slits 142, with the fourth stripes 141 corresponding to thesecond slits 122 of the second region 120. Here, the first stripes 111and the third stripes 131 are parallel to a first direction, and thesecond stripes 121 and the fourth stripes 141 are parallel to a seconddirection perpendicular to the first direction. The stripes do not needto be exactly parallel, but only need to have sufficient parallelalignment to accomplish the desired result.

The multiple first to fourth stripes 111, 121, 131 and 141 of the firstto fourth regions 110, 120, 130 and 140 are made of a materialsufficiently opaque to shield a laser beam. The multiple first to fourthslits 112, 122, 132 and 142 are sufficiently transparent so as totransmit a laser beam. It is desirable that the widths of the multiplefirst to fourth stripes 111, 121, 131 and 141 be smaller than or equalto those of the multiple first to fourth slits 112, 122, 132 and 142.This allows an amorphous silicon film to be completely exposed to thelaser beam using the subsequently described inventive process. Thewidths of the multiple first to fourth stripes 111, 121, 131 and 141 canbe changed according to the energy density of the laser beam oraccording to the condition of the silicon film. For example, the widthsof the stripes or slits can be within a range of about 2 mm to about 10mm. Sub-ranges can be defined at 4 mm, 6 mm and 8 mm widths.

The mask 100 of FIG. 6 shows a particular arranging order of the firstto fourth regions 110, 120, 130 and 140, but that order can be varied.For example, the mask 100 could be arranged in order of (from right toleft) the first region 110, the third region 130, the second region 120,and the fourth region 140. The widths of the first to fourth regions110, 120, 130 and 140 are preferably equal.

FIGS. 7A to 7D show schematic plane views of a process for crystallizingan amorphous silicon film according to an embodiment of the invention.

In FIG. 7A, a silicon film 200 crystallizes by a first irradiation of alaser beam through the mask 100 (of FIG. 6). The laser beam irradiatesonto first portions 210 of the silicon film 200 that correspond to themultiple first slits 112 (of FIG. 6) in the first region 110 (of FIG. 6)of the mask 100. The first portions 210 melt and then cool to form firstgrains 211 and 212. The growth of the first grains 211 and 212 startsfrom the edges of the first portions 210 that are exposed to the laserbeam, and the growth progresses toward the center of the first portions210 to stop at a line “Va” where grains having different growingdirections meet.

In FIG. 7B, after the silicon film 200 having the first portions 210crystallized by the first irradiation is moved by a quarter width of themask 100 (of FIG. 6) to the left, a second irradiation of the laser beamis performed onto second portions 220 of the silicon film 200. As aresult, the second portions 220 correspond to the multiple second slits122 (of FIG. 6) in the second region 120 (of FIG. 6) of the mask 100.The second portions 220 melt and then cool, resulting in crystallizationof the silicon film 200. In the second portions 220, second grains 221and 222 grow from edges of the second portions 220 in a directionperpendicular to a first grain-growth direction of the first grains 211and 212 (of FIG. 7A). The second grains 211 and 212 stop growing at aline “Vb” where the second grains 211 and 212 meet. In regions where thefirst and second portions 210 and 220 overlap, the first grains 211 and212 by the first irradiation act as crystallization seeds. Thus, thesecond grains 221 and 222 are larger than the first grains 211 and 212.

In FIG. 7C, after the silicon film 200 having the first and secondportions 210 and 220 (of FIGS. 7A and 7B) crystallized by the first andsecond irradiations is moved a quarter width of the mask 100 (of FIG. 6)to the left, a third irradiation of the laser beam is performed ontothird portions 230 of the silicon film 200. The third portions 230correspond to the multiple third slits 132 (of FIG. 6) in the thirdregion 130 (of FIG. 6) of the mask 100. The third portions 230 melt andthen cool such that crystallization occurs in the third portions 230. Inthe regions where the second portions 220 and the third portions 230overlap, the second grains 221 and 222 by the second irradiation act ascrystallization seeds. Thus, third grains 231 and 232 by the thirdirradiation are enlarged.

In FIG. 7D, after the silicon film 200 having the first to thirdportions 210, 220 and 230 (of FIGS. 7A to 70) crystallized by the firstto third irradiations is moved by a quarter width of the mask 100 (ofFIG. 6) to the left, a fourth irradiation of the laser beam is performedon fourth portions 240 of the silicon film 200. The fourth portions 240correspond to the multiple fourth slits 142 (of FIG. 6) in the fourthregion 140 (of FIG. 6) of the mask 100. The fourth portions 240 melt andthen cool such that crystallization occurs. In regions where the thirdand fourth portions 230 and 240 overlap, the third grains 231 and 232 bythe third irradiation act as crystallization seeds. Therefore, fourthgrains 241 and 242 having a square shape are obtained.

By repeating the foregoing process, a polycrystalline silicon filmhaving large and relatively uniform grains is created. However, sincethe polycrystalline silicon film also has a grain boundary (sides of thesquare shape), a TFT fabricated by using the polycrystalline siliconfilm has relatively inferior characteristics when a channel of the TFTincludes the grain boundary. Accordingly, when the TFT is fabricatedsuch that the channel does not include the grain boundary, improvedcharacteristics of the TFT can be obtained.

FIGS. 8 and 9 show schematic views of a position of a thin filmtransistor according to first and second embodiments of the invention,respectively.

FIGS. 8 and 9 show a thin film transistor (TFT) “T” that is formed on asubstrate 300 by using the polycrystalline silicon film of FIG. 7D as anactive layer. The polycrystalline silicon film has grains of squareshape, i.e., grain boundaries are disposed only at sides of the squareshape, and the inner portion of the square shape is single crystallinesilicon. Accordingly, if the TFT “T” forms having a channel “ch” smallerthan the grain, the channel “ch” can be disposed inside of the grain. Asa result, when the channel “ch” forms having the width and length 340and 320 smaller than the two sides 310 and 330 of the grain, the TFT “T”can have a channel “ch” where no grain boundary exits.

TFTs of FIGS. 8 and 9 are disposed along different directionsperpendicular to each other. As a result, the channels of the TFTs formin the respective polycrystalline silicon films having different latticedirections. However, since the channels of the TFTs form in one grain,the channels have no grain boundary. Therefore, the TFTs of FIGS. 8 and9 do not have differences in characteristics due to the dispositiondirection of the channels.

FIG. 10 shows a schematic plane view of an array substrate for a liquidcrystal display device according to an embodiment of the invention.

In FIG. 10, an array substrate 400 for an LCD device includes a displayregion “L” and a peripheral region “M” surrounding the display region“L.” Gate and data lines 410 and 420 and a switching TFT “T subs” usingpolycrystalline silicon are formed in the display region “L.” A drivecircuit 450 and gate and data drivers 430 and 440 are formed in theperipheral region “M.” The switching TFT “T.sub.s” connects to the gateand data lines 410 and 420. The drive circuit 450 includes multipledriving TFTs (not shown) using polycrystalline silicon. Each of theswitching TFT “T.sub.s” and the driving TFTs includes an active layer, agate electrode and source and drain electrodes. The active layer is madeof polycrystalline silicon having grains of square shape. The switchingTFT “T.sub.s” and the driving TFTs are disposed such that all channelsof the TFTs are formed inside the grains. Therefore, the switching TFTand the driving TFTs using polycrystalline silicon have similarcharacteristics to a transistor using single crystalline silicon.

To form the channel of the TFT inside the grain, one desires that thegrain be enlarged or the size of the channel be reduced. Moreover, analignment key (not shown) for the forming process of the active layer,which is formed during the crystallization process, aligns the channelof the TFT inside the grain. In addition, the TFTs of the drive circuit450 are arranged according to the position of the grain when the drivecircuit and the photo mask are designed.

Consequently, the drive circuit including CMOS devices simultaneouslyforms on the array substrate to simplify the fabrication process andreduce production costs. Furthermore, since the channel of the TFT isformed inside the grain, the channel has no grain boundary, and the TFThas no resulting property variation in accordance with the direction ofdisposition. Moreover, since the grain is nearly a single crystal, theproperty of the TFT is similar to that of a transistor using singlecrystalline silicon, such as produced from a wafer. Therefore, a gatedriver and a data driver are simultaneously formed on a substrate with apixel TFT. Various drive circuits such as a complex CPU (central processunit) can be integrated in one body according to developments ofexposure technology.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the liquid crystal displaydevice and the fabricating method thereof of the invention withoutdeparting from the spirit or scope of the inventions. Thus, it isintended that the invention covers the modifications and variations ofthis invention provided they come within the scope of the appendedclaims and their equivalents.

1. A thin film transistor for a liquid crystal display device,comprising: a substrate; an active layer over the substrate, the activelayer including polycrystalline silicon having square shaped grains; agate electrode overlapping the active layer to form a channel region,the channel region being formed inside one of the grains; and source anddrain electrodes connected to both sides of the active layer, whereinone end of the gate electrode is disposed inside the one of the grains,and the other end of the gate electrode has a size greater than the oneof the grains to occupy at least two grains.
 2. The thin film transistoraccording to claim 1, wherein the polycrystalline silicon is obtained bycrystallizing amorphous silicon with a laser beam.
 3. A thin filmtransistor, comprising: a substrate; an active layer over the substrate,the active layer including polycrystalline silicon grains; a gateelectrode overlapping the active layer to form a channel region, thechannel region being formed inside one of the grains; and source anddrain electrodes connected to both sides of the active layer, whereinone end of the gate electrode is disposed inside the one of the grains,and the other end of the gate electrode has a size greater than the oneof the grains to occupy at least two grains.
 4. The thin film transistoraccording to claim 3, wherein the grains are square.